Polymerization
The
process of converting a monomer or a mix of monomers into a polymer is named polymerization.
It is a process during which relatively small molecules, called monomers,
chemically combine to make a really large chainlike or network molecule called
a polymer. Monomer molecules can all be the same or they can represent two,
three, or more different compounds. Typically, to make a product you need to
combine at least 100 monomer-molecules that have some unique physical
properties such as elasticity, high tensile strength, or the ability to form
fibers that separate polymers from substances made up of small and simple
molecules; Often, several thousand monomer units are integrated into a single
molecule of polymer. The formation of stable covalent chemical bonds between
monomers set polymerization in addition to other processes like
crystallization, where large amounts of molecules combine under the influence
of weak intermolecular energy.
The
two classes of polymerization are usually distinguished. In condensation
polymerization, water is often present with the formation of molecules of
several common compounds at each step of the process. In addition
polymerization, monomers react to polymer formation without forming
by-products. Addition polymerizations are usually conducted in the presence of
catalysts, which in some cases control the structural details that have a
significant effect on the properties of the polymer.
Mechanism of polymerization
The
mechanism of polymerization process utilized in polymer synthesis allows
the next classification into two main classes, namely, addition and
condensation polymers. The former type is produced by the repetition and sequential
addition of monomers without damaging a small molecule during the process.
Therefore, no by-products are formed and the repeating units of additional
polymers have the same formula that was used to make them, like alkenes or
virtually-substituted alkene monomers. These addition reactions follow a step-by-step system involving reactive mediators such as radicals or ions that
help to convert monomer pi bonds into polymer sigma bonds. The four different
polymerization techniques used in the synthesis of additive polymers are (i)
radical polymerization, (ii) cationic polymerization, (iii) anionic
polymerization, and (iv) coordination catalytic polymerization.
The
process of radical polymerization follows three steps, initiation,
propagation, and termination. During initiation, a molecule called the radical
initiator splits into a thermal or photolytically free radical. A radical
alkene then attacks the pi bond in the monomer to form a covalent bond with one
of the carbon atoms and turns the other into an active one. The propagation
stage then becomes a chain by continuously adding more monomers later on.
Termination of chain growth eventually occurs when radical chains combine or
participate in the unnecessary reactions involved by pulling hydrogen from another
radical chain. Cationic and anionic polymerization follows a similar path
overall as their promoters are strong acids and Lewis acids (to convert an
alkene into cation), or strong bases, alkaline metals, and organolithium
compounds (to convert an alkyne monomer to anion). Catalytic polymerization,
known as the Ziegler-Natta polymerization technique, employs catalysts that are
complex transition metal-based synthesis complexes derived from transition
metal halides and organometallic reagents.
The second class of polymers consisting of a large number of highly useful
materials is those produced by the polycondensation of monomers combined with a
single structure. Condensation polymers are bi-functional in monomers that
result in the conversion of functional groups so that each monomer can connect
with two others. Polymerization usually leads to the loss of a small molecule
that can be in the form of water, gas, or salt. A well-known condensation
polymer is Nylon 6,6, a polyamide made from adipic acid (a dicarboxylic
acid) and 1,6-hexamethylenediamine, a combination of which destroys water
molecules. Nylon 6,6 finds numerous applications in the production of clothing,
cooking utensils, carpets, fishing lines, and much more. As polycondensation
tends to be slower than polyaddition, the reaction often needs to be heated. A
direct consequence of slow polymerization is usually the formation of low
molecular-weight polymers. The concentration of high crystals with high tensile
strength results from strong interchain interactions when polar functional
groups are present along the chain.
Degree of polymerization
The
degree of polymerization can be defined as the frequency of the
repetition units present in the polymer. For example, if a polymer (p) is
composed of 5 numbers of monomers (M), its polymerization degree will be 5.
Further,
you'll calculate its degree using the steps mentioned below-
You
must first enter the chemical formula of the polymer. For example, consider
tetrafluoroethylene [- (CF2-CF2) n -]. The first bound
element refers to the monomer unit.
Next,
you need to collect the atomic mass of an element that forms a monomer. In this
case, carbon and fluorine are involved. From the periodic table, you need to
examine the atomic mass of these two elements. The atomic masses of fluorine
and carbon are 19 and 12, respectively.
Calculate
the molecular weight of the monomer using the following steps.
To calculate the molecular weight, you need to multiply the atomic mass by the
number proportional to the quality of the atomic mass (carbon or fluorine
atom).
Add
both products to get molecular weight. For tetrafluoroethylene, it's (19 x 4) +
(12 x 2) = 100.
Finally,
you need to divide the molecular mass of the polymer by the calculated
molecular weight of the monomer. For example, the polymerization degree
of tetrafluoroethylene would be 1200 if its molecular number was 120,000.
Thus,
the degree of polymerization formula can be defined as the ratio between
the molecular mass of the polymer and the molecular weight of the monomer.
Some examples of a polymerization
N-Vinylpyrrolidone (NVP)
An
organic compound N-vinyl pyrrolidone (NVP) containing a 5-member lactam linked
with a vinyl group. It is a colorless liquid although commercial samples may be yellow in color. It is produced by industrial-based 2-pyrrolidone vinyl, i.e. a
base-catalytic reaction with acetylene. It is a precursor to an important
synthetic material polyvinylpyrrolidone (PVP). NVP monomers are commonly used
as a reactive dilute between ultraviolet and electron-beam curing polymers
applied as ink, coating, and adhesive.
p-methyl styrene polymerization
The
synthesis of stereoregular styrenic polymers in single-site catalysts has
received considerable attention in recent decades; However, fully isotactic
poly (P-methyl styrene) is rarely known for tailoring. It has been shown here
that the isospecific coordination polymerization of p-methyl styrene can be
achieved in the presence of the catalyst
dichloro[1,4-dithiabutanediyl-2,20-bis(4,6-di-tert-butyl-phenoxy)] titanium
activated by methyl aluminoxane. Furthermore, p-methyl styrene has been assumed
to be non-crystallized for decades, as X-ray diffraction and differential
scanning calorimeter measurements of the powder sample failed to reveal a
well-defined crystal structure. However, dendritic crystals of P-methyl styrene
were successfully made from dilute solutions by carefully controlling the
evaporation rate of the solvent. Crystal morphology was studied by optical
microscopy and atomic force microscopy. In situ heating tests of dendritic
crystals allow us to measure the melting temperature of p-methyl styrene
crystals. In addition, a vapor annealing process is performed to prepare
multilamellar crystals for X-ray scattering measurements at small angles to
mark the spacing and orientation of the formed crystalline lamellae.
N-carboxy anhydride polymerization
N-carboxy
anhydride (NCA) polymerization is the most widely used polymerization technique
to make synthetic polypeptides and polypeptide-based block copolymers on the
multigram scale. NCA polymerizations were initiated using a variety of
different nucleophiles and bases, the most common being primary amines and
alkoxide anions. The primary amines, being more nucleophilic than the basics,
are good general initiators for the polymerization of NCA monomers. Compared to
nucleophilic, tertiary amines, alkoxides, and other precursors have found use
because they are able to prepare very high-molecular-weight polymers in some
cases where the primary amine cannot initialize. The optimal polymerization
conditions are often determined faithfully for each NCA and so there is no
universal indicator or condition that can produce a high polymer from any
monomer. This is partly due to the different properties of the individual NCAs
and their polymers (e.g., solubility) but is strongly related to the side
effects that occur during polymerization.
Xylan polymerization
Xylan
is a type of hemicellulose that represents the third most abundant biopolymer
in the world. It is found in plants, dicot second cell walls, and all cell
walls of grass. Xylans are polysaccharides produced from β-1,4-linked xylose
residues with side branches of α-arabinofuranose or α-glucuronic acids that in
some cases contribute to the crucible connection with cellulose microfibrils
and lignin through ferulic acid residues. Based on the substituted group, Xylan
can be classified into three classes i) Glucuronoxylan (GX) ii) Neutral
Arabinoxylan (AX), and iii) Glucuronoarabinoxylan (GAX).
Xylan
plays an important role in plant cell wall integrity and enhances cell wall
recovery in enzymatic digestion; Thus, they help to protect plants from herbs
and germs. Xylan also plays an important role in the growth and development of
trees. The quality of cereal flour and the hardness of the dough are largely
influenced by the amount of xylan, which plays an important role in the bread
industry. The main ingredient in xylan can be converted to xylitol used as a
natural food sweetener, which helps reduce tooth decay and acts as a sugar
substitute for diabetics.
N-vinylcarbazole polymerization
N-vinylcarbazole
is an organic compound used as a monomer in the manufacture of polyvinyl
carbazole, a conductive polymer, where the conductivity is photon-dependent.
The compound is used in the photoreceptors of photocopiers. Upon contact with
γ-irradiation, N-vinyl carbazole undergoes solid-state polymerization. It is
produced by the vinylation of carbazole with acetylene in the presence of the
base.
A
crystal of N-vinyl carbazole was polymerized in water suspended by a redox
catalyst and polyvinyl carbazole was found. Polymerization progresses rapidly
above 40°C without solid-state bringing time. The molecular weight of the
polymer increased with a decrease in catalytic density and an increase in
temperature. Observations of partially polymerized crystals through a
polarizing microscope showed that the polymerization progressed beyond the surface
of the monomer crystal and that birefringence was observed in the polymer
layer. X-ray compression studies showed that the polymer was crystalline.
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